"Tis" is a contraction of "It is". The phrase "Tis the season" is part of an old and beloved Christmas song:
Deck the halls with boughs of holly,
Fa la la la la, la la la la.
'Tis the season to be jolly,
Fa la la la la, la la la la.
Don we now our gay apparel
Fa la la, la la la, la la la
Troll the ancient yuletide carol
Fa la la la la, la la la la!
This is an old song with a lot of antiquidated expressions; a non-rhyming modern version might be
Decorate your house with holly branches,
Fa la la la la, la la la la.
It's time to be happy,
Fa la la la la, la la la la.
We're putting on our party clothes,
Fa la la, la la la, la la la
Sing the old Christmas song,
Fa la la la la, la la la la!